Bagiennik was the name of water demons in the Slavic mythology. They were akin to the bathhouse spirit Bannik. They were subject to Wąda, lady of the lakes and the shallow streams, also known as the Queen of the Underwater Lawns.

The Bagienniks lived in the depths of lakes and rivers. They were capable of emitting an oily substance from the nostrils, which were located between the eyes or on the forehead. This substance was so hot that it burned the victims it hit, but it was also provided with notable healing abilities for rheumas, deep wounds, indigestion, heart illness and even infertility.

The presence of bagienniks could be detected by the bubbles on the water's surface, or by its darker and muddy colour.